The subject matter of this disclosure relates to providing building cooling, dehumidification, and fresh air ventilation through a range of outdoor and indoor conditions.
New U.S. homes that are built in compliance with ASHRAE Standard 90.2, Energy Star, and other energy efficiency programs have lower cooling loads than in the past, and because they are of tighter construction, they frequently require mechanical ventilation as prescribed by ASHRAE Standard 62.2. In humid climates, the ventilation air often requires more dehumidification than can typically be provided by air conditioners, because typical air conditioners in energy-efficient homes have short run times during many cooling load hours. Short run times typically limit latent cooling capacity, since less air passes over the cold evaporator coil. Failure to control excessive indoor humidity has contributed to problems with indoor mold. This issue has become increasingly expensive for homeowners and builders as mold-related property damage and class action lawsuits have risen steadily.
Vapor compression cooling systems (air conditioners) that are in use in most homes and other buildings provide a mix of sensible cooling (lowering the air temperature) and latent cooling (removing moisture). Typically, the sensible heat ratio (“SHR”, the sensible cooling capacity divided by the total cooling capacity) for most residential cooling systems ranges from 0.7 to 0.8. In humid conditions, this SHR is often too high to maintain temperature and relative humidity in the ideal ranges of 74°-78° F., and 40-60%, respectively. Some vapor compression cooling systems lower the airflow rate through the evaporator coil to reduce the SHR under humid conditions, but re-evaporation of condensate retained on the coil at system shutdown still limits the SHR, particularly when systems cycle frequently, as they do under low load conditions. Such residential cooling systems are “split systems”, with an outdoor condensing unit that includes the compressor, condensing coil, and condenser fan, and a separate indoor unit that includes an evaporator coil, expansion device, and system blower. Two refrigerant lines join the outdoor and indoor components.
Furthermore, a stand-alone dehumidifier is frequently used in humid climates to control indoor humidity. Because heat from the condenser is added to indoor air, the dehumidifier often increases the sensible cooling load, the air conditioner run time, and overall energy consumption. A preferred approach to dehumidification in the cooling season is to dehumidify indoor air by rejecting condenser heat to outside air instead of to the indoor space.
In the prior art, various strategies have been proposed to control both temperature and humidity. For example, U.S. Pat. No. 6,170,271 B1 shows a concept with two separate refrigerant loops: a first loop with the evaporator in the supply air stream and the condenser outdoors, for sensibly cooling the air stream; and a second “latent cooling” loop with the evaporator just downstream of the first evaporator, and with the condenser downstream of the second evaporator. This approach is similar to combining an air conditioner and a dehumidifier, but with the added benefits of requiring only one indoor blower and cabinet. A smaller second evaporator can be used because the air has been pre-cooled in the first evaporator. However, all heat from the second loop is added to the supply air, with associated energy penalties. In the embodiment, having the dehumidifier condenser located outside the supply air stream, the system is still penalized by the cost of requiring dual compressors, additional refrigerant piping, and condensers. Various other design configurations appear in the patent literature and are aimed at more precisely controlling both sensible and latent loads.
Another strategy having dual refrigerant loops is shown in U.S. Pat. No. 6,705,093 B1, which uses two condensing units that share an evaporator coil whose tubing pattern maintains separation of the two loops. One of the two loops has a sub-cooling coil. This approach adds substantial cost to a conventional system with a single refrigerant loop. Another approach to increasing latent cooling is shown in U.S. Pat. No. 6,427,454 B1. This design selectively causes a portion of the return air to bypass the evaporator coil, which lowers the coil temperature and increases moisture condensation on the coil. However, this approach is unlikely to succeed in the market, as it is comparable to lowering the blower speed, but with higher initial costs and without the energy savings associated with reducing blower speed.
U.S. Pat. No. 6,123,147 shows a retrofit system that adds a hot water reheat coil connected to a residential water heater located downstream of the evaporator. Like other “reheat” designs, this approach decreases the SHR by making the cooling system run longer. However, the economics of such a system will be poor because gas water heating is substituted for waste heat already available from the condensing side of the refrigerant system. Thus, this approach is like driving a vehicle using the accelerator and brake simultaneously. Other strategies, such as that disclosed in U.S. Pat. No. 5,791,153, apply desiccant-based enthalpy wheels to increase latent cooling. These designs require added components to recharge the desiccant and therefore may not be cost-effective.
Of the major product lines in the U.S. marketplace, only the Carrier® Infinity™ series and the Lennox™ SignatureStat™ controller claim features that control both temperature and humidity.
In the “packaged” air conditioning market with products usually applied to non-residential buildings, Lennox™ markets a patented Humiditrol® line that includes refrigerant control valves and a “hot gas” reheat coil for more precise humidity control. Carrier® markets the MoistureMiser™ that uses a “sub-cooling” coil for the same purpose. In both cases the strategy is to add some heat from the condenser side of the refrigerant system back into the supply air stream (downstream of the evaporator) to reduce the net cooling rate, as well as reducing supply air relative humidity. Such systems must run longer to satisfy the cooling load, and the longer run time removes more moisture at the evaporator. Adding more length to the coil on the condenser side also reduces the liquid refrigerant temperature into the evaporator, which increases evaporator capacity and therefore drops the evaporator temperature, increasing the rate of moisture removal. Lennox™ claims superior dehumidification performance because the higher heat output of the “hot gas” approach causes longer cooling cycles, thus removing more moisture compared to the sub-cooling approach.
These non-residential products use a “single-package” configuration, and no “split system” units currently include the “reheat coil” features described above. In fact, the Lennox™ hot gas approach is only workable in a single package device, as the system would require an extra pair of refrigerant lines to be applied in a “split system” configuration because refrigerant must flow first to the indoor reheat coil, then back to the condenser, then to the indoor expansion device. The Carrier® sub-cooling approach would not require an extra line set in a split system configuration because the refrigerant flows directly from the sub-cooling coil to the expansion device. However, the approach only provides two stages of dehumidification, and therefore cannot sufficiently control humidity when sensible loads are very low and latent loads are high.